The development of high energy battery systems requires the compatibility of an electrolyte possessing desirable electrochemical properties with highly reactive anode materials, such as lithium, sodium and the like, and the efficient use of high energy density cathode materials, such as manganese dioxide. The use of aqueous electrolytes is precluded in these systems since the anode materials are sufficiently active to react with water chemically. It has, therefore, been necessary, in order to realize the high energy density obtainable through use of these highly reactive anodes and high energy density cathodes, to turn to the investigation of nonaqueous electrolyte systems and more particularly to nonaqueous electrolyte systems based on organic solvents.
The term "nonaqueous electrolyte" in the prior art refers to an electrolyte which is composed of a solute, for example, a salt or a complex salt of Group I-A, Group II-A or Group III-A elements of the Periodic Table, dissolved in an appropriate nonaqueous organic solvent. Conventional solvents include propylene carbonate, ethylene carbonate or .gamma.-(gamma)butyrolactone. The term "Periodic Table" as used herein refers to the Periodic Table of the Elements as set forth on the inside front cover of the Handbook of Chemistry and Physics, 63rd Edition, CRC Press Inc., Boca Raton, Fla., 1982-1983.
Although manganese dioxide has been mentioned as a cathode for cell applications, manganese dioxide inherently contains an unacceptable amount of water, both of the adsorbed and bound (absorbed) types, which is sufficient to cause anode (lithium) corrosion along with its associated hydrogen evolution. This type of corrosion that causes gas evolution is a serious problem in sealed cells, particularly in miniature type button cells. In order to maintain battery-powered electronic devices as compact as possible, the electronic devices are usually designed with cavities to accommodate the miniature cells as their power source. The cavities are usually made so that a cell can be snugly positioned therein thus making electronic contact with appropriate terminals within the device. A major potential problem in the use of cell-powered devices of this nature is that if the gas evolution causes the cell to bulge then the cell could become wedged within the cavity. This could result in damage to the device. Also, if electrolyte leaks from the cell it could cause damage to the device. It is therefore important that the physical dimensions of the cell's housing remain constant during discharge and that the cell will not leak any electrolyte into the device being powered.
In order to reduce the water content in manganese dioxide, several processes have been developed. For example, U.S. Pat. No. 4,133,856 discloses a process for producing an MnO.sub.2 electrode (cathode) for nonaqueous cells whereby the MnO.sub.2 is initially heated within a range of 350.degree. C. to 430.degree. C. so as to substantially remove both the adsorbed and bound water and then, after being formed into an electrode with a conductive agent and binder, it is further heated in a range of 200.degree. C. to 350.degree. C. prior to its assembly into a cell. British Pat. No. 1,199,426 also discloses the heat treatment of MnO.sub.2 in air at 250.degree. C. to 450.degree. C. to substantially remove its water component.
U.S. Pat. No. 4,285,122 discloses a process whereby a homogeneous mass of particulate manganese dioxide is heat-treated and then contacted with an organic solvent that substantially fills the pores of the manganese dioxide with a layer of the organic solvent which effectively decreases the affinity or propensity of the manganese dioxide for readsorbing moisture.
U.S. Pat. No. 4,379,817 discloses a process whereby the walls of the pores of manganese dioxide are coated by vapor-depositing an organic solvent thereon to reduce the manganese dioxide's affinity for adsorbing moisture when exposed to a moisture-containing environment for a fixed time period.
Although manganese dioxide with reduced water content is better suited for nonaqueous cell systems, it was noted that cells employing this type of active material had a tendency to show increased internal impedance during storage. This condition is accompanied by poor closed circuit voltage, poor high and low temperature shelf life, poor cell voltage maintenance characteristics, and poor pulse rate and discharge capabilities.
U.S. patent application Ser. No. 447,106 filed Dec. 6, 1982 discloses a nonaqueous cell employing a solid cathode comprising manganese dioxide, a binder, a conductive agent and at least one compound selected from the group consisting of alkaline earth metal hydroxides such as Mg(OH).sub.2, Ca(OH).sub.2, Ba(OH).sub.2 and Sr(OH).sub.2 and alkaline earth metal carbonates, such as MgCO.sub.3, CaCO.sub.3, BaCO.sub.3 and SrCO.sub.3 to suppress or minimize the increase in the internal impedance of the cell which may occur during storage or discharge.
It is an object of the present invention to provide a new class of additives for manganese dioxide-containing cathodes intended for use in nonaqueous cells that will suppress or minimize the increase in the internal impedance of the cell which may occur during storage or discharge.
Another object of the present invention is to provide a nonaqueous cell employing among other components a manganese dioxide-containing solid cathode having a minor amount of an additive such as lithium metasilicate (Li.sub.2 SiO.sub.3), lithium tetraborate (Li.sub.2 B.sub.4 O.sub.7), lithium molybdate (Li.sub.2 MoO.sub.4) or lithium orthophosphate (Li.sub.3 PO.sub.4) to improve the voltage maintenance and pulse discharge characteristics of the cell even after prolonged storage.